The present invention relates to a cylinder head structure of an engine, and specifically a technical field of a cylinder head structure in which a water jacket circulating an engine cooling water therein is provided in a cylinder head.
Normally, an engine body comprises a cylinder block and a cylinder head which is attached on the cylinder block. In a multi-cylinder engine, there are generally provided bolt holes at four side corners and both sides of the cylinder head. The cylinder head is fastened to the cylinder block via head bolts being inserted into these bolt holes. The bolt holes at the both sides of the cylinder head are located between adjacent cylinders. In this structure, in order to prevent a so-called gas leakage between the adjacent cylinders and improve the sealing, a sealing means such as a gasket is provided between the upper face of the cylinder block and the lower face of the cylinder head.
However, a portion located between adjacent cylinders of the cylinder head is fastened to a portion of the cylinder block which corresponds to the portion located between cylinders of the cylinder head via the above-described blots located at the both sides. Accordingly, a central portion of the above-described portion located between cylinders of the cylinder head is apt to be raised upward slightly due to the thermal expansion of the cylinder caused by heating. As a result, the face pressure at the portion against the upper face of the cylinder block may decrease, thereby deteriorating the sealing between them. Particularly, this deterioration of sealing may be critical to a compact engine with a small distance between adjacent cylinders.
Japanese Utility Model Laid-Open Publication No. 59-99150 discloses a cylinder head structure equipped with a reinforcing rib which is provided at the portion located between cylinders, as a countermeasure for the above-described problem. Herein, this reinforcing rib is provided so as to connect the central portion of the portion located between cylinders with boss portions of bolt holes which are formed at both sides of this portion. Namely, the rigidity of the portion located between cylinders may be increased by the reinforcing rib connecting the central portion of the portion with the boss portions of the bolt holes, and any improper deformation due to the thermal expansion may be restrained thereby.
Meanwhile, the water jacket for cooling the engine body including the cylinder head is generally provided in the cylinder head. Herein, it is required for a connecting hole provided for interconnecting the water jacket formed in the cylinder block and the water jacket formed in the cylinder head to be disposed in a so limited space in the cylinder head.
It is considered, for example, that such connecting holes are located at the above-described portion located between cylinders and at its outer side in the engine width direction respectively. In this case, however, it may not be easy to connect the central portion of the portion located between cylinders and the boss portions at both sides by the reinforcing ribs like the structure disclosed in the above-described patent document.
Also, it may be necessary to further increase the rigidity of the cylinder head in the cylinder-line direction as well as in the vertical direction, in order to properly prevent the gas leakage between adjacent cylinders of the cylinder head which includes many holes such as the water jackets.